Display screen or portion thereof with a graphical user interface



The FIGURE is a front view of a display screen or portion thereof with a graphical user interface showing the new design.

The broken lines showing a display screen or portion thereof and elements of the graphical user interface illustrate portions of the article that form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 